Light emitting diode (LED) technology is a maturing technology that continues to show improvements in efficiency, customability and cost reduction. LED technology is rapidly being deployed in a host of industries and markets including general lighting for homes, offices, and transportation, solid state display lighting such as in LCDs, aviation, agricultural, medical, and other fields of application. The increased energy efficiency of LED technology compared with other lighting solutions coupled with the reduction of costs of LED themselves are increasing the number of LED applications and rate of adoptions across industries. While LED technology promises greater reliability, longer lifetimes and greater efficiencies than other lighting technologies, the ability to mix and independently drive different color LEDs to produce customized and dynamic light output makes LED technology and solid state lighting (SSL) in general robust platforms to meet the demands of a variety of market needs and opens the door to many new applications of these lighting technologies. The ability to tailor and tune the output spectra of LED fixtures and dynamically switch individual LEDs “on-the-fly”, for example in response to an environmental cue, dramatically opens up the application space of solid state lighting.
As is well known in the art, LED luminaires generally comprise one or more individual LEDs dies or packages mounted on a circuit board. The LEDs may be electrically connected together on a single channel or be distributed and electrically driven across multiple independent channels. The LEDs are typically powered by current from an associated LED driver or power supply. Examples of these power supply drivers include AC/DC and DC/DC switched mode power supplies (SMPS). Examples of LED power drivers include power supplies designed to supply constant current to the LED string in order to maintain a consistent and steady light output from the LEDs. LEDs may also be powered by an AC power source. Direct AC power typically undergoes rectification and other power conditioning prior to being deliver to the LEDs. LED luminaires may also comprise an optic or diffuser, a heat sink and other structural components.
Although LEDs may be combined in such a way to deliver a wide variety of specific color outputs, LED luminaires for general lighting typically are designed to produce white light. Light perceived as white or near-white may be generated by a combination of red, green, and blue (RGB) LEDs. Output color of such a device may be altered by color mixing, for instance varying the amount of illumination produced by each of the respective color LEDs by adjusting the supply of current to each of the red, green, and blue LEDs. Another method for generating white or near-white light is by using a lumiphor such as a phosphor in conjunction with a blue “pump” LED. Still another approach for producing white light is to stimulate phosphors or dyes of multiple colors with an LED source. Many other approaches can also be taken.
Correlated Color Temperature (CCT), measured in degrees Kelvin (K), is a common a metric to characterize broad band light sources. CCT was introduced to address broadband light sources that may not be modeled by a blackbody radiator. CCT is defined as the temperature of a blackbody radiator whose chromaticity point is closest to the chromaticity point of the non-planckian light source. Every illumination source has a (radiometric) spectral power distribution whose output can be expressed as the integral of radiant power over the wavelength range of the light-emitting source. The eye's perception of this source can be expressed as a single chromaticity value, an ordered pair in a planar color-space (CCx, CCy), according to CIE1931 color space diagram. Other color spaces exist.
FIG. 1 is an example CIE 1931 diagram that illustrates, inter alia, the planar color-space with associated set of coordinates (x,y) representing perceived colors. The perceived color of any light source can be defined as a location on the color space. Individual LEDs are typically characterized by chromaticity (i.e., an x, y coordinate pair in the CIE color space) and luminous flux (φ=Y) weighted by the luminous efficiency function (Vλ). To create white light from multiple LED sources with varying wavelengths and intensities, LEDs may be mixed such that the resulting output matches a specific coordinate on the color-space plane.
FIG. 2 shows example spectral power distributions (SPDs) from conventional white light LEDs of three different correlated color temperatures. For each of these white light LED sources, the peak at around 450 nm represents the light contribution from a blue “pump” LED and the broader peak, for example and light above 500 nm, is due to the luminescence of one or more phosphors that have been excited by the blue light. In these conventional LED white light sources there is a trough of spectral power in the region around 490 nm.
LEDs, as with all manufactured products, have material and process variations that yield products with corresponding variation in performance. At present, LED manufacturers are challenged to produce uniform color points in their white LEDs and are limited to a “bandwidth spread” in their monochromatic LEDs as well. There are a number of reasons for this inability to achieve mass production of LEDs with uniform color points, key among them t are related to the packaging of the LEDs. There may be considerable variability from LED to LED, particularly in the case of phosphor converted LEDs, since both the variability of the LED chip and the phosphor coating can introduce variability into the performance of the final packaged LED. While the manufacturers of the packaged LEDs typically “bin” the final packaged LEDs to provide products of similar light and color output, even LEDs in the same bin will exhibit variations in color output.
Additionally, the light conversion efficiency of a specific LED and any associated phosphor coating may depend on the temperature at which the LED operates and how the LED is driven electrically. Differently packaged LEDs, even those within the same bin and that have the same light output at one temperature and drive current, may have different light output at other temperatures and/or drive currents. In many circumstances, until the packages are assembled into an operational luminaire or lighting device, the extent of any such variability cannot be fully determined.
Although embodiments of the invention are not dependent on such, it is believed that the gap in spectral power output between 480 and 500 nm, with a trough around 490 nm, that exists in conventional white light LEDs (e.g., as shown in FIG. 2) is a result of the LED industry recognizing the challenges posed in color uniformity when employing light in the aforementioned region. The retinal response over this region (e.g., 480-500 nm), is such that the eye and visual system is extremely discriminative of light and light color in this spectral region. For example, and as can be seen in FIG. 1, the CIE color space diagram, the variation in perceived color, as represented by the variation in color points over this 20 nm range between 480 nm and 500 nm is relatively large, for instance when compared with the perceived color changes in the region of 440 nm to 460 nm.
Additionally, LED manufacturers who make monochromatic LEDs, with a Full Width Half Maximum (FWHM) less than 40 nm, can typically only guarantee that any LED of a specific bin (i.e., within a certain color spectral bandwidth) will vary by no more than 5 nm in color output from another LED of the same bin. A lighting designer or manufacture attempting to construct a luminaire with a specific color output spectrum is challenged to provide a luminaire with consistent color output while using LEDs which may have an unacceptable wide range (e.g., 5 nm) of light output. Hence, because of the enhanced visual discrimination in the 480-500 nm color region, employing monochromatic LEDs in this region may result in unacceptable perceived color differences between LED fixtures that are designed to yield the same color output. Generating an LED spectrum with a consistent (x,y) color point while using monochromatic enhancement in the region from 480 nm-500 nm is a problematic challenge.
Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins, and is found in intrinsically photosensitive retinal ganglion cells (ipRGCs) of humans and other mammals. Melanopsin plays an important non-image-forming role in the photoentrainment of circadian rhythms as well as potentially many other physiologic functions. Stimulation of melanopsin-containing ipRGCs contributes to various reflexive responses of the brain and body to the presence of light. FIG. 3 shows the action spectrum of melanopsin 30 together with SPDs of conventional LED lights of different color temperatures 32. Melanopsin photoreceptors are sensitive to a range of wavelengths and reach peak light absorption at wavelengths around 480-500 (or 490) nanometers (nm). Recent scientific studies have shown that 480-500 nm light (the region of melanopic-producing light) is very important for non-visual stimuli including physiological and neuroligcal effects such as pupillary light reflex and circadian entrainment. Conventional LED lighting fixtures provide less than optimal and potentially insufficient light in these biologically important wavelength ranges (e.g., non-visual stimulus) at standard light levels.
Blue Light Hazard”, as defined by ANSI/IESNA RP-27.3-07, is the potential for a photochemically induced retinal injury resulting from radiation exposure primarily between 400 nm and 500 nm. Scientific data indicates that blue light can cause excessive amounts of reactive oxygen species in the retina, which may result in cumulative oxidative stress which can cause inter alia accelerated cellular aging in the retina. FIG. 3 illustrates the spectral region 34 associated with the blue light hazard. Even with conventional light levels, blue light exposure may cause long term damage over the course of years of exposure. This oxidative stress may be compounded and/or accelerated if the lighting illumination spectrum is deficient or depleted of light associated with non-visual stimulus. For example, the pupillary light reflex (PLR) is a reflex that controls the diameter of the pupil in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the eye. This reflex thereby assists in, inter alia, adaptation to various levels of lightness or darkness. Insufficient stimulus of the RGCs, which may occur in the absence of sufficient melanopic light, that is light that falls within the melanopsin action spectrum region as shown in FIG. 3 and which provides the necessary stimulus of the RGCs, may result in reduced pupillary constriction, thereby allowing more blue light to enter the eye potentially resulting in increased and accelerated oxidative stress on the retina.
There is a need for general lighting device that delivers white light with excellent color rendering and esthetic characteristics and provides sufficient flux of melanopic light and generates sufficient spectral power in the relevant wavelengths to provide adequate non-visual stimulus associated with important physiological responses and functions. There is a need for lighting that reduces oxidative stress on the retina that results from blue light exposure.
In view of the enhanced human visual sensitivity in the 480-500 nm region and the inherent binning limitations of LEDs packages and the associated variability of color output of these LEDs, there is a need for methods for achieving and lighting devices that achieve consistent color temperature and color points while providing light of adequate or optimal melanopic flux.